The present invention relates generally to a color cathode ray tube and more particularly to an in-line type color cathode ray tube having an in-line type electron gun structure, wherein convergence characteristics of electron beams emitted from the in-line type electron gun structure are improved.
In general, an in-line type color cathode ray tube, as shown in FIG. 1, has an envelope comprising a panel 1 and a funnel 2 formed continuous with the panel 1. An inner surface of the panel 1 is provided with a phosphor screen 3 composed of three-color phosphor layers emitting red (R), green (G) and blue (B) respectively. A shadow mask 4 is disposed adjacent, and opposed to, the phosphor screen 3.
A neck 5 of the funnel 2 of the color cathode ray tube includes an in-line type electron gun structure for emitting three electron beams which are lined side by side on a horizontal axis, i.e. an X-axis, as shown in FIG. 2. Specifically, the electron gun structure emits a center beam directed to the green phosphor layer of the phosphor screen 3 and a pair of side beams directed to the red and blue phosphor layers of the phosphor screen 3.
In addition, the color cathode ray tube, as shown in FIG. 1, has a deflector 6 mounted on the outer peripheral surface of a portion extending between the funnel 2 and neck 5. A two-pole magnet 7 having a pair of an N-pole and an S-pole, which are opposed to each other, is disposed at a rear end portion of the deflector 6. The two-pole magnet 7 is used to adjust landing of electron beams.
A convergence magnet 8 is disposed on the outside of the neck 5. The convergence magnet 8 has at least a ring-shaped four-pole magnet plate 11 and a ring-shaped six-pole magnet plate 10. The four-pole magnet plate 11 has two pairs of N-poles and S-poles which are opposed to each other. The six-pole magnet plate 10 has three pairs of N-poles and S-poles which are opposed to each other.
At the time of non-deflection, the two-pole magnet 7 and convergence magnet 8 function to register the three electron beams, emitted from the electron gun structure, at a center of the phosphor screen 3, thereby achieving high color purity and convergence.
In the color cathode ray tube, the three electron beams emitted from the electron gun structure are deflected by a non-homogeneous magnetic field produced by the deflector 6 and scanned over the phosphor screen. Thus a color image is reproduced on the phosphor screen 3.
In the in-line type color cathode ray tube, the electron beams are susceptible to an external magnetic field such as earth magnetism. The conditions of external magnetic field vary when the picture tube is situated in use in a direction different from the direction in which the convergence adjustment was made, or when the picture tube is used on a location where the condition of earth magnetism differs from that at a place of adjustment. Consequently, there may arise a problem in that a red image and a blue image displayed on the phosphor screen by a pair of side beams are vertically displaced from each other. The theory of occurrence of this phenomenon is as follows.
According to Jpn. Pat. Appln. KOKAI Publication No. 7-250335, an electron gun structure is disposed within the neck, as described above. The electron gun structure has a cathode which is heated by a heater to emit thermal electrons. The cathode is formed of a low-thermal-expansion material, i.e. magnetic material. For example, when an external static magnetic field such as earth magnetism has intersected with a tube axis or a Z-axis of the neck portion in a use environment, the external magnetic field is converged toward the cathode or magnetic body. Consequently, magnetic fields in horizontally opposite directions act on the paired side beams, in particular, of the three electron beams. These mutually opposite magnetic fields exert mutually opposed forces to the side beams.
In other words, the external magnetic field has horizontal components or X-axis components in mutually opposed directions with respect to the side beams. For example, when an external magnetic field in a positive direction along the X-axis acts on the electron beam for red, a force acts in a vertically downward direction, i.e. in a negative direction along the Y-axis and the electron beam for read shifts in the negative direction along the Y-axis. On the other hand, an external magnetic field in a negative direction along the X-axis acts on the electron beam for blue. Thus, a force acts in a positive direction along the Y-axis and the electron beam for blue shifts in the positive direction along the Y-axis. Consequently, a red image and a blue image displayed on the phosphor screen by the pair of side beams are vertically displaced relative to each other.
According to the idea disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-21938, if three electron beams are converged, a pair of side beams have magnetic field components opposite to each other in the X-axis direction. If an external magnetic field in the Z-axis direction is applied in this state, images displayed by the side beams will be vertically displaced relative to each other due to Lorentz force as described above.
In order to prevent displacement of images displayed by the side beams, a pair of magnetic bodies 9 for shielding the Z-directional external magnetic field are disposed, as shown in FIG. 2. The magnetic bodies 9 extend along the Z-axis and are situated on both sides of the neck 5 in the X-axis direction.
The magnetic bodies 9 are normally fixed in the Z-direction on the inner surfaces of a cylindrical holder H of the convergence magnet 8, as shown in FIG. 2, in order to reduce the number of fixation steps and to control the precision in fixation.
On the other hand, the six-pole magnet plate 10, as shown in FIG. 3, has three N-poles and three S-poles equidistantly arranged on the ring-shaped magnet plate. These poles are alternately arranged and produce a magnetic field distribution, as shown in FIG. 3. According to this distribution, a force in the same direction is applied to the pair of side beams and varies the trajectories. On the other hand, the magnetic field intensity is canceled and set at substantially zero on the trajectory of the center beam, i.e. the center axis of the color cathode ray tube, and no force acts to vary the trajectory.
If the convergence magnet for projecting the static magnetic field for correcting the trajectories of three electron beams and the magnetic bodies for shielding the external magnetic field are arranged within the limited dimensions of the neck portion, as described above, part of the strip-like magnetic bodies intersects with part of the ring-shaped magnet plate.
If the magnetic bodies and magnet plate are arranged close to each other, the magnetic bodies are magnetized by the function of the magnet plate, in particular, the magnetic poles of the six-pole magnet plate. As a result, the following problem will occur.
FIGS. 4A and 4B show a distribution of a magnetic field produced by the six-pole magnet plate when the trajectories of both side beams of the three electron beams are to be corrected in the positive direction along the Y-axis, and the state in which the magnetic bodies are magnetized.
In this case, the six-pole magnet plate 10 is situated such that one of the N-poles and one of the S-poles are positioned on the X-axis. At this time, portions of the magnetic bodies 9a and 9b arranged opposite to each other on the X-axis are situated near the N-pole and S-pole of the six-pole magnet plate 10. Thus, the portions of the magnetic bodies 9a and 9b, which are situated near the poles of the six-pole magnet plate 10, are magnetized with polarities opposite to those of the adjacent poles. The entire magnetic bodies are magnetized in its length direction, i.e. the Z-axis direction. As a result, two-pole magnetic fields are produced at the front end portions of the magnetic bodies, i.e. the end portions near the magnet plate, and the rear end portions of the magnetic bodies.
Specifically, an S-pole is produced on that surface of the magnetic body 9a located on the positive (+) side of the X-axis, which is in contact with the N-pole of the magnet plate 10, and an N-pole is produced at the front and rear end portions of the magnetic body 9a. Similarly, an N-pole is produced on that surface of the magnetic body 9b located on the negative (-) side of the X-axis, which is in contact with the S-pole of the magnet plate 10, and an S-pole is produced at the front and rear end portions of the magnetic body 9b.
Thereby, a magnetic field directed from the magnetic body 9a to magnetic body 9b, i.e. a negative magnetic field component directed from the (+) side to (-) side along the X-axis, is produced at the front and rear end portions of the magnetic bodies 9a and 9b. This magnetic field component exerts an upward force to the electron beam passing by the rear end portion of the magnetic body.
In addition, near the surface of the six-pole magnet plate 10, magnetic fluxes of the poles positioned on the X-axis are guided to the magnetic bodies 9a and 9b. As a result, the negative magnetic field component produced by the magnet plate 10 in the direction from the (+) side to (-) side on the X-axis is weakened. As mentioned above, the magnet plate 10 is designed such that the magnetic field intensity on the trajectory of the center beam becomes zero due to the balance in magnetic field between the two poles on the X-axis and the four poles on the Y-axis in the state in which the magnetic bodies are not disposed. However, when the magnetic bodies are disposed, the magnetic field produced by the two poles on the X-axis is guided by the magnetic bodies and weakened. As a result, the positive magnetic field component produced by the four poles of magnet plate 10 near the Y-axis in the direction from the (-) side to (+) side on the X-axis is relatively increased.
More specifically, in the vicinity of the front end portions of the magnetic bodies, as in the vicinity of the rear end portions, the negative magnetic field component directed from the (+) side to (-) side on the X-axis is produced. However, since the positive magnetic field component produced by the four poles near the Y-axis in the direction from the (-) side to (+) side on the X-axis is relatively strong, a positive magnetic field component is produced as a total magnetic field on the trajectory of the center beam.
In other words, the negative magnetic field component is produced on the trajectories of the side beams near the magnet plate 10, and the positive magnetic field component is produced on the trajectory of the center beam. The direction of the magnetic field on the trajectories of the side beams is opposite to the direction of the magnetic field on the trajectory of the center beam.
As has been described above, as regards the magnetic fields which the respective electron beams emitted from the cathode 16 receive until they travel the deflector on the respective trajectories, a positive magnetic field as a whole acts on the trajectory of the center beam and a negative magnetic field as a whole acts on the trajectories of the side beams. Thus, the side beams passing through the plane of the six-pole magnet plate receive force in the positive direction on the Y-axis and the center beam receives force in the negative direction on the Y-axis.
As a result, as regards the magnet plate wherein when the electron beam trajectory is to be corrected without the provision of the magnetic bodies the center beam is not shifted and both side beams can be shifted by 1.3 mm to the (+) side on the Y-axis, if the magnetic bodies are mounted on the magnet plate, both side beams are moved to the (+) side by 0.5 mm on the Y-axis and the center beam is moved to the (-) side on the Y-axis by 0.8 mm.
This degrades the operability of the magnet plate. Moreover, the center beam moves at the time when the beam trajectory is corrected by the six-pole magnet plate after landing adjustment was effected by the two-pole magnet plate. Consequently, the landing adjustment needs to be effected once again by the two-pole magnet, degrading the efficiency of the adjustment work.
As stated above, when the electron beam trajectory is vertically corrected in the state the magnetic bodies are disposed, such problems will arise that the amount of movement of both side beams decreases and the center beam moves in a direction opposite to the direction of movement of the side beams.